Given, $W = \frac{1}{2}kx^{2}$ Writing the dimensions on both sides $\ \ \ \ \left\lbrack ML^{2}T^{- 2} \right\rbrack = \ k\left\lbrack M^{0}L^{2}T^{0} \right\rbrack$ $\therefore$ Dimensions of $k = \left\lbrack MT^{- 2} \right\rbrack = \left\lbrack {ML}^{0}T^{- 2} \right\rbrack$